Sens. Rob Portman (R-Ohio), Shelley Moore Capito (R. W.Va.), Cory Gardner (R-Colo.) and Lisa Murkowski (R-Alaska) said they would vote against any reform bill that fails to protect Americans who became eligible for coverage under the ACA’s expansion of Medicaid, the government-run health program for the poor and disabled.

All four senators represent states that opted to expand their Medicaid program under the ACA.

“We will not support a plan that does not include stability for Medicaid expansion populations or flexibility for states,” they wrote in a letter to Senate Majority Leader Mitch McConnell (R-Ky.)

“Reform should not come at the cost of disruption in access to health care for our country’s most vulnerable and sickest individuals. Any changes made to how Medicaid is financed through the state and federal governments should be coupled with significant new flexibility so they can efficiently and effectively manage their Medicaid programs to best meet their own needs.”